KR20140086496A - The method of controlling switch relectance motor and apparatus using the same - Google Patents

Abstract

Disclosed are a method of controlling a switched reluctance motor (SRM) and an apparatus using the same. The method of controlling the driving of a switched reluctance motor comprises the steps of detecting a change in load of the SRM; and controlling both a dwell angle of the SRM and pulse width modulation (PWM) duty according to a change in load of the SRM. Accordingly, torque ripples can be reduced at a low speed, torque can be controlled at a high speed, and a prompt response performance for a command speed can be ensured by controlling an angle and a current at the same time. Further, a speed of the SRM and a torque can be controlled more precisely.

Description

[0001] The present invention relates to an SRM control method and a device using such a method,

The present invention relates to SRM control methods and apparatus using such methods.

Switched reluctance motors are one of the oldest motor designs over 150 years old. This type of conventional reluctance motor has become known as a switched reluctance motor to meet the requirements of a variable drive with the development of power semiconductors. The name "Switched Reluctance" Named by Nasar, this name describes two major features of SRM. First, 'Switched' means that the motor must always be operated in continuous switching mode, a term that has been used since the development and development of new types of power semiconductors. Second, 'Reluctance' refers to a dual-pole-type structure in which the rotor and the stator operate by varying the reluctance magnetic circuits.

 Nasra, French, Koch, Lawrenson, etc., contemplate continuous mode control using power semiconductors in the 1960s, unlike structurally similar stepping motors. At that time, the power thyristor semiconductor had the ability to control only the relatively high voltage and current, which was applied to control the switched reluctance motor. Modern power transistors, GTOs, IGBTs, and power MOSFETs have been developed and are now available in a wide range of rated power for SRM control.

SRM is very simple in terms of structure. There is no permanent magnet, no brush, no commutator, the stator has a stacked structure of steel with salient poles, the windings of the coils wound in series are connected independently to the respective phases, It has a wrapping structure. The rotor has no winding, has a laminated structure of steel, and has a pole-like structure like the stator. Therefore, both the stator and the rotor have a salient pole structure, which can be regarded as a double salient structure. Such a simple structure can be said to be an alternative to a variable speed drive because reliability and production cost are reduced.

The patent literature described in the following prior art documents, KR2002-0003781, relates to a rotor position detection method of a switched reluctance motor. Specifically, a method of measuring the rotor position of an SRM without attaching an additional position sensor such as an encoder has been disclosed.

KR 2002-0003781

One aspect of the present invention relates to a method of controlling the driving of an SRM according to a load variation.

Another aspect of the present invention relates to a drive control apparatus for controlling the drive of the SRM according to a load variation.

The apparatus for controlling a switched reluctance motor (SRM) according to an embodiment of the present invention includes a load sensing unit for sensing a load variation of the SRM, a dwell angle and PWM duty ratio of the SRM according to a load variation of the SRM, (pulse width modulation duty ratio). The load variation of the SRM may be a revolution per minute (rpm) change of the SRM. When increasing the rpm of the SRM, the controller may increase the dwell angle and the PWM duty ratio of the SRM by a predetermined ratio. The controller may be configured to reduce the dwell angle and the PWM duty ratio of the SRM at a constant rate when decreasing the rpm of the SRM. Wherein the load sensing unit is configured to determine whether the rpm variation of the SRM is a specific threshold value and if the rpm variation exceeds the specific threshold value, the dwell angle and the PWM duty ratio of the SRM, May be implemented to increase or decrease a certain rate.

According to another aspect of the present invention, there is provided a method of driving a switched reluctance motor (SRM), including the steps of sensing a load variation of the SRM, calculating a dwell angle and a PWM duty ratio (pulse width modulation duty ratio). The step of sensing the load variation of the SRM may be a step of sensing a revolution per minute (rpm) change of the SRM. The step of controlling both the dwell angle and the PWM duty ratio of the SRM according to the load variation may be a step of increasing the dwell angle and the PWM duty ratio of the SRM by a predetermined ratio when the rpm of the SRM is increased. The step of controlling both the dwell angle and the PWM duty ratio of the SRM according to the rpm variation may include reducing the dwell angle and the PWM duty ratio of the SRM at a constant rate when decreasing the rpm of the SRM . The step of controlling both the dwell angle and the pulse width modulation duty ratio of the SRM according to the rpm variation may include determining whether the rpm variation of the SRM is a specific threshold value, And increasing or decreasing the dwell angle and the PWM duty ratio of the SRM by a predetermined ratio in accordance with the variation of the rpm when the predetermined threshold value is exceeded.

The features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.

Prior to that, terms and words used in the present specification and claims should not be construed in a conventional and dictionary sense, and the inventor may properly define the concept of the term in order to best explain its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

As described above, the SRM control method and the apparatus using the SRM control method according to the present invention detect the load variation of the SRM and detect the dwell angle of the SRM and the PWM duty ratio modulation duty ratio can be controlled. Therefore, torque ripple reduction at low speed, constant torque control at high speed, and fast response performance to command speed can be ensured by simultaneously controlling angle and current. In addition, the speed and torque of the SRM can be controlled more precisely.

1 is a conceptual view showing a structure of an SRM according to an embodiment of the present invention.
2 is a conceptual diagram illustrating inductance and torque according to the position of a rotor in an SRM according to an embodiment of the present invention.
3 is a conceptual diagram illustrating a method for controlling load fluctuation in an SRM according to an embodiment of the present invention.
4 is a conceptual diagram illustrating an SRM control method according to an embodiment of the present invention.
5 is a conceptual diagram illustrating an SRM control method according to an embodiment of the present invention.
6 is a conceptual diagram illustrating an SRM control method according to an embodiment of the present invention.
7 is a flowchart illustrating an SRM control method according to an embodiment of the present invention.
8 is a flowchart showing an SRM control apparatus according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The objectives, specific advantages and novel features of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: FIG. It should be noted that, in the present specification, the reference numerals are added to the constituent elements of the drawings, and the same constituent elements are assigned the same number as much as possible even if they are displayed on different drawings. It is also to be understood that the terms "first,"" second, "" one side,"" other, "and the like are used to distinguish one element from another, no. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the following description of the present invention, detailed description of related arts which may unnecessarily obscure the gist of the present invention will be omitted.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

 1 is a conceptual view showing a structure of an SRM according to an embodiment of the present invention.

In FIG. 1, SRMs having a dual-pit structure are shown, in which the number of poles of a stator pole 100 and a rotor pole 120 is 8/6. The material of the stator 100 and the rotor 120 may be made of iron having a high magnetic permeability and may have a structure capable of being laminated by a stratified layer. The windings are wound on the stator.

The driving characteristic of the switched reluctance motor will be briefly described. When the stator pole of the SRM is excited, the rotor 120 corresponding thereto is induced to the stator 100 excited to minimize the reluctance of the magnetic circuit. Therefore, when the rotor 120 approaches the corresponding stator 100, a constant torque can be obtained by exciting the windings of the next stator 100 continuously.

That is, when a current flows on the stator 100, a torque is generated to rotate the rotor 120 in a direction in which the inductance increases until the rotor 120 reaches a position having a maximum inductance value. If there is no magnetization component in the iron core, the direction of the current is independent of the polarity of the torque, and the torque is always generated in the direction to go to the nearest aligned position.

The polarity of the torque generated according to the position of the rotor when the current continues to flow during one period of the inductance will be described in detail in Fig.

According to the embodiment of the present invention, when a load variation (for example, a fluctuation of RPM) is detected in the SRM, the dwell angle and the pulse width modulation duty ratio are controlled together at a certain ratio A method for quickly responding to a load variation of the SRM is disclosed.

(1) The dwell angle indicates the position of the rotor where the stator current is switched on in the SRM, the turn-on angle, and the rotor position where the high electron current is switched off The term "turn off angle" refers to the difference between the turn off angle and the turn on angle. That is, the dwell angle may be a period in which the switch of the inverter is turned on. By varying the dwell angle, it is possible to respond to the load fluctuation of the SRM.

(2) PWM is a method of controlling the overall average voltage using the pulse width. The pulse may be a waveform in which a high signal and a low signal cross each other at a predetermined timing. The ratio of the length of the high signal to the length of the low signal is referred to as a duty ratio. When the duty ratio is high, since the high signal becomes long, the average voltage may rise. For example, a PWM duty ratio of 5% means that if the period is 100 seconds, it has a high signal for 5 seconds and a low signal for the remaining 95 seconds.

In the embodiment of the present invention, a method of responding to the load variation of the SRM based on the dwell angle and the PWM duty ratio will be described below.

2 is a conceptual diagram illustrating inductance and torque according to the position of a rotor in an SRM according to an embodiment of the present invention.

와

는 고정자와 회전자의 극호(pole-arc)이다. 한 상의 인덕턴스 곡선의 주기는 이웃한 두 회전자 극 사이의 각도, 회전자극 피치(

)와 같고 이것이 인덕턴스 프로파일의 주기가 된다.Figure 2a shows the position of the rotor in the SRM.

Wow

Is the pole-arc of the stator and rotor. The period of the inductance curve of one phase is determined by the angle between the two neighboring rotor poles,

), Which is the period of the inductance profile.

In case of 8/6 motor, the inductance curve of each phase varies with phase difference by 15 degrees. Therefore, in order to rotate the motor continuously, it is necessary to make the torque to be generated in a certain direction by sequentially switching the current only in the section where the inductance of each phase increases.

Referring to FIG. 2C, the torque is proportional to the square of the current, so that torque can be generated irrespective of the direction of the phase current. Depending on the rate of change of the inductance, the torque causes a positive torque or a negative torque .

As shown in FIG. 2B, the inductance increases, decreases or has a constant interval. If a constant excitation current is allowed to flow through the phase winding, a constant torque is generated in the section where the inductance increases, and a sub torque is generated in the section where the constant torque is decreased.

Therefore, when a constant exciter is applied to the SRM, the static torque and the sub torque are canceled each other, and the torque of the electric motor becomes zero, so that the rotational torque can not be obtained. Therefore, it is necessary to detect the position angle of the rotor in order to prevent the generation of the sub torque and obtain the effective rotation torque, and to perform the switching operation according to the position angle. That is, an ideal torque must be generated by flowing a switching excitation current to each phase of the SRM.

3 is a conceptual diagram illustrating a method for controlling load fluctuation in an SRM according to an embodiment of the present invention.

3 shows the inductance profile according to the position angle of the rotor and the excitation current waveform due to the switching of the stator yarn.

According to the embodiment of the present invention, the excitation current waveform can be changed using a method of changing the dwell angle and the pulse width modulation duty together in a method of controlling the load fluctuation in the SRM.

For example, the dwell angle and the PWM duty ratio can be varied at a constant rate.

For example, assuming that the PWM duty ratio varies between 10 and 100% and the dowel angle is between 60 and 15 degrees, the variation width of the PWM duty ratio becomes 90% and the variation width of the dowel angle can be 45 degrees. In this case, when the PWM duty ratio changes by 1 percent, the dwell angle can be changed by 0.5 degrees to control the SRM load.

When the dwell angle and the PWM duty ratio change, the following changes may occur in the SRM.

)은 권선에 전원을 인가하여 여자(magnetizing)하는 구간이다. 진상각이 변화할 경우, 턴온 시점이 앞당겨져 전류 상승 시간이 달라지게 된다. 드웰각과 진상각을 변화시켜 SRM의 rpm(revolution per minute)을 조절할 수 있다. 예를 들어, 진상각을 조절하여 턴 온 지점을 앞당겨 충분한 전류 상승 시간을 도모하고, 드웰각을 조절하여 토크 발생 영역을 최대한 이용하되, 역 토크가 발생되는 구간에 도달하기 전에 전류의 크기를 최소화시켜 역 토크 발생을 억제할 수 있다. 즉, 드웰각이 조절되는 경우, 토크 발생 영역을 최대한 이용하되 역 토크가 발생되는 구간에 도달하기 전에 전류의 크기가 최소화할 수 있다.1) Dowel angle ( ) Is the difference between the turn-off angle and the turn-off angle as described above. When the rotor position where the stator current is switched on is referred to as a turn on angle and the rotor position where the rotor current is switched off is referred to as a turn off angle You can tell the difference between turn off angle and turn on angle. Phase angle

) Is a section for magnetizing by applying power to the windings. When the phase angle is changed, the turn-on time is advanced and the current rise time is changed. The rpm (revolution per minute) of the SRM can be adjusted by changing the dwell angle and phase angle. For example, by adjusting the phase angle to advance the turn-on point, sufficient current rise time can be achieved and the dowel angle can be adjusted to maximize the torque generation region. However, before the reverse torque is reached, So that generation of reverse torque can be suppressed. That is, when the dowel angle is adjusted, the magnitude of the current can be minimized before the torque generating region reaches the region where the torque generating region is generated.

In addition, since the torque characteristic of the SRM is independent of the current direction and becomes equal to the sign of the slope of the inductance, it is impossible to rotate the SRM in the opposite direction by controlling only the current. It is necessary to allow the current to flow in the section generating the torque. In addition, angle control can be used when sudden braking is desired.

In other words, when the dwell angle changes, the torque fluctuation period in the SRM changes, and the load fluctuation of the SRM can be controlled.

2) When the PWM duty ratio is changed, the load current fluctuation of SRM can be controlled by controlling the current flowing through SRM. The method of controlling the load variation of the SRM by changing the PWM duty ratio can be mainly used to control the SRM driven at low speed and medium speed.

In the case of SRM driven at low speed or medium speed, since the back electromotive force and the increase of the inductance of the motor occur slowly, the peak current can be larger than that at the high speed operation because the rate of increase of the current due to the applied voltage is large. To limit this current to less than the current of the switching element, the switching element can be turned on and off by chopping to control the SRM at the desired speed.

That is, when the dwell angle and the PWM duty ratio, which are the two SRM load control elements, change together, the load variation of the SRM can be more effectively controlled. By controlling the angle and current simultaneously, torque ripple reduction at low speed, constant torque control at high speed, and fast response performance to command speed can be ensured. In addition, the speed and torque of the SRM can be controlled more precisely.

Also in accordance with the embodiment of the present invention, a method of controlling both of the two control elements to control the load variation can be used in combination with a method of controlling the operation of the SRM using one control element. Embodiments of the present invention will now be described with reference to these methods.

4 is a conceptual diagram illustrating an SRM control method according to an embodiment of the present invention.

FIG. 4 shows a method of controlling the operation of the SRM by controlling both the dwell angle and the PWM duty ratio when the RPM of the SRM is increased by increasing only the dwell angle to control the load of the SRM.

Referring to FIG. 4A, the RPM of the SRM may be increased from 500 to 1000 and from 1000 to 1500. FIG. A section is the section where the RPM of the SRM operates at 500, section B is the section at which the RPM of the SRM operates at 1000, and section C is the section at which the RPM of the SRM operates at 1500.

For example, when the RPM of the SRM is 1000 or more, the dwell angle and the PWM duty ratio are both controlled to control the RPM using only the dowel angle as the SRM control variable and the specific SRM The control method for controlling the operation of the SRM based on the RPM of the SRM can be selected differently.

4B is a conceptual diagram showing a current waveform according to an operation interval.

1) A period, as shown in FIG. The current waveform (b) when the RPM of the SRM increases from 500 rpm to 1000 rpm can increase the rpm of the SRM by increasing the dwell angle and increasing the effective torque generation period than the current waveform (a) when operating at 500 rpm .

2) the current waveform (c) in the C section. When the rpm of the SRM increases from 1000 rpm to 1500 rpm, the value of the phase current and the dowel angle can be adjusted to increase the effective torque generation period. That is, when the RPM of the SRM is sensed and the rpm value of the specific SRM is exceeded, the load variation of the SRM can be controlled by using both control variables.

In FIG. 4, although the specific RPM is described as a condition for changing the control variable, the control variable for controlling the operation of the SRM may be changed under a condition other than the specific RPM, and such an embodiment is also included in the scope of the present invention.

Also, although FIG. 4 exemplifies only the case where the rpm of the SRM increases, the embodiment as described above can also be applied to the case where the rpm of the SRM is reduced. That is, when the rpm decreases, the number of control variables that control the SRM legacy can be reduced by one control variable (dwell angle or PWM duty ratio) in the two control variables (dwell angle and PWM duty ratio).

As an example, the above embodiment controls the operation of the SRM through the PWM duty ratio and controls the operation of the SRM using both the dwell angle and the PWM duty ratio when the specific condition is satisfied.

5 is a conceptual diagram illustrating an SRM control method according to an embodiment of the present invention.

5, a method of controlling the operation of the SRM by controlling both the PWM duty ratio and the dwell angle when the rpm of the SRM is increased by increasing the PWM duty ratio only when the specific duty rpm is exceeded in controlling the operation of the SRM . It is also possible to control the operation of the SRM using control variables other than the RPM of the SRM.

Referring to FIG. 5A, the RPM of the SRM can be increased from 500 to 1000, and from 1000 to 1500. FIG. A section is the section where the RPM of the SRM operates at 500, section B is the section at which the RPM of the SRM operates at 1000, and section C is the section at which the RPM of the SRM operates at 1500.

For example, if the RPM of the SRM is controlled by controlling the RPM of the SRM by controlling only the PWM duty ratio at the RPM of less than 1000, and controlling the dwell angle and the PWM duty ratio of the RPM of the SRM by 1000 or more, Different control methods can be selected as a basis.

5B is a conceptual diagram showing a current waveform according to an operation interval.

1) A period, as shown in FIG. The current waveform (b) when the RPM of the SRM increases from 500 rpm to 1000 rpm increases the rpm of the SRM by increasing the value of the phase current than the current waveform (a) when operating at 500 rpm and increasing the effective torque generation period .

2) the current waveform (c) in the C section. When the rpm of the SRM increases from 1000 rpm to 1500 rpm, the effective torque generation period can be increased by adjusting the value of the phase current and the dowel angle. That is, when the RPM of the SRM is sensed and the rpm value of the specific SRM is exceeded, the load variation of the SRM can be controlled by using both control variables.

In FIG. 5, control variables for controlling the operation of the SRM may be changed under conditions other than the RPM of the SRM, and such embodiments are also included in the scope of the present invention.

6 is a conceptual diagram illustrating an SRM control method according to an embodiment of the present invention.

FIG. 6 shows a method of controlling the operation of the SRM by changing the PWM duty ratio and the dwell angle at a constant rate in the embodiment of FIG.

6A, when the rpm of the SRM decreases from 1500 to 1300 (G section) and the rpm of the SRM increases from 1500 to 1800, the PWM duty ratio and the dwell angle are changed at a constant ratio The rpm of the SRM can be controlled.

Referring to FIG. 6B, in the case of the G section, the PWM duty ratio and the dowel angle can be reduced at a constant rate. When the PWM duty ratio and the dwell angle are reduced at a constant rate, the phase current can have the current waveform g, thereby reducing the RPM of the SRM.

Conversely, in the case of the H section, the PWM duty ratio and the dowel angle can be increased at a constant rate. When the PWM duty ratio and the dwell angle are increased at a constant rate, the phase current can have the current waveform h, thereby increasing the RPM of the SRM.

7 is a flowchart illustrating an SRM control method according to an embodiment of the present invention.

FIG. 7 shows a method of controlling the rpm of the SRM by increasing or decreasing the dwell angle and the PWM duty ratio at a constant rate according to the rpm of the SRM.

Referring to FIG. 7, it is determined whether to change the rpm of the SRM (step S700).

The value of the rpm of the SRM can be measured by the rpm sensing unit and it can be determined whether to increase or decrease the value of the rpm so that the dwell angle and the PWM duty ratio can be increased or decreased at a constant rate. When two control elements, the dwell angle and the PWM duty ratio change together, the SRM load fluctuation can be more effectively controlled. By controlling the angle and current simultaneously, torque ripple reduction at low speed, constant torque control at high speed, and fast response performance to command speed can be ensured. In addition, the speed and torque of the SRM can be controlled more precisely.

When the rpm of the SRM is increased, the dwell angle and the PWM duty ratio are increased at a constant rate (step S710).

As described above, for example, the variation width (for example, 10 to 100%) at which the PWM duty ratio changes and the variation width (for example, 60 to 15 degrees) at the dowel angle can be increased at a constant rate.

When the rpm of the SRM is reduced, the dwell angle and the PWM duty ratio are reduced at a constant rate (step S720).

In the case of controlling both the dwell angle and the PWM duty ratio only when the specific rpm is more than or less than a specific rpm, and controlling only one of the dwell angle and the PWM duty ratio, the rpm of the SRM to be controlled before the step shown in FIG. Value to determine whether to control the SRM by using any control variable.

8 is a flowchart showing an SRM control apparatus according to an embodiment of the present invention.

Referring to FIG. 8, the SRM controller may include a load determination unit 800, a control variable determination unit 820, and a control unit 840.

Each component will be represented separately according to the function for convenience of explanation. Each constituent part may be divided into a plurality of constituent parts or a plurality of constituent parts may be combined into one constituent part.

The load determining unit 800 determines the rpm at which the SRM currently operates or determines the rpm to be operated by the SRM and transmits the rpm information to be operated to the control variable determining unit 820. When the control variable determining unit 840 determines that the SRM May be determined.

The control variable determining unit 840 can determine which control variable is to be controlled based on the current SRM operation information. As in the above example, it is decided to control the operation of the SRM using only one control variable (dwell angle or PWM duty ratio) below a certain rpm, and when two or more control variables (dwell angle and PWM duty ratio) are exceeded All of which can be controlled to control the operation of the SRM.

The control unit 840 may control the operation of the SRM by changing the control variable determined by the control variable determining unit 820 as a part for controlling the operation of the SRM. In the SRM driving control method according to the embodiment of the present invention, the driving of the SRM can be controlled by changing both of the two control variables (dwell angle and PWM duty ratio) at a constant rate.

If the SRM controls the driving of the SRM in such a manner that both the dwell angle and the PWM duty ratio are controlled, the control variable determining unit 820 may be used separately and only the load determining unit 800 and the controlling unit 840 may be included. Embodiments are also within the scope of the present invention.

While the present invention has been described in detail with reference to the exemplary embodiments, it is to be understood that the present invention is not limited thereto and that the present invention is not limited thereto. It is evident that it can be modified or improved. That is, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

100: stator
120: rotor
800:
820: Control variable determining unit
840:

Claims (10)

A load sensing unit for sensing a load variation of the SRM; And
And a controller for controlling both the dwell angle and the pulse width modulation duty ratio of the SRM according to the load variation of the SRM. 2. The method of claim 1,
Wherein the SRM is a revolution per minute (rpm) change of the SRM. 3. The apparatus of claim 2,
Wherein when the rpm of the SRM is increased, the dwell angle and the PWM duty ratio of the SRM are increased at a constant rate. 3. The apparatus of claim 2,
And when the rpm of the SRM is reduced, the dwell angle and the PWM duty ratio of the SRM are reduced at a constant rate. 3. The method of claim 2,
The load sensing unit is configured to determine whether the rpm variation of the SRM is a specific threshold value
Wherein the controller is configured to increase or decrease the dwell angle and the PWM duty ratio of the SRM by a predetermined ratio in accordance with the variation of the rpm when the rpm variation exceeds the specific threshold value, Sensing a load variation of the SRM; And
And controlling both the dwell angle and the pulse width modulation duty ratio of the SRM according to the load variation of the SRM. The method of claim 6, further comprising: sensing a load variation of the SRM
And detecting a rpm change of the SRM. 7. The method of claim 6, wherein controlling both the dwell angle and the PWM duty ratio of the SRM according to the load variation comprises:
And increasing the dwell angle and the PWM duty ratio of the SRM at a constant rate when increasing the rpm of the SRM. 7. The method of claim 6, wherein controlling both the dwell angle and the PWM duty ratio of the SRM according to the rpm variation comprises:
Wherein when the rpm of the SRM is reduced, the dwell angle and the PWM duty ratio of the SRM are reduced at a constant rate. 7. The method of claim 6, wherein controlling both the dwell angle and the pulse width modulation duty ratio of the SRM according to the rpm variation comprises:
Determining whether the rpm variation of the SRM is a specific threshold value;
And increasing or decreasing a dwell angle and a PWM duty ratio of the SRM by a predetermined ratio in accordance with the variation of the rpm when the rpm variation exceeds the specific threshold value.

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